Conventionally, in the fields of architecture and civil engineering, friction joining is generally conducted in which fastening devices such as high-strength bolts are used to fasten steel products to be joined, which are then mutually joined by utilizing a frictional resistance resulting from the compressive force derived from the fastening devices, thereby providing a joining structure for steel products constituting a framework of steel structures (buildings, bridges and the like). In general friction joining, steel products to be joined such as base materials (columns, beams, braces and the like) or joining members (splice plates, gusset plates, and the like) are processed to form the friction surface by the following methods to secure a friction coefficient. That is, sanding or grinding is applied to remove black scale from the steel products, which are then allowed to stand to develop red rust, or shot blasting is applied to make the friction surface rough. However, these methods only generate a relatively small friction coefficient on the friction surface and also have difficulty in securing a stable frictional resistance. Thereby, there is no choice but to adopt a lower value, with consideration given to safety in design, making it difficult to provide a rational design. Under these circumstances, it has been long desired to solve the above difficulty.
Meanwhile, it is known that a pair of joining faces which are in contact with each other for steel products to be joined are processed so that they are mutually different in hardness and roughness as a structure for increasing a frictional resistance between steel products to be joined by friction joining (refer to Japanese Published Unexamined Patent Application No. 2002-155910, for example). The friction-joining structure described in Patent Document 1 is such that one of the joining faces is subjected to one-time shot blasting and the other joining face is subjected to two-time shot blasting, by which the joining faces are made into friction surfaces different in hardness and roughness, thereby increasing the friction coefficient between the friction surfaces and reducing the number of bolts or making the diameter small, and realizing a rational design.
However, in a conventional friction-joining structure described in the above-described Patent Document 1, since joining faces of steel products to be joined are subjected to one-time or multiple-time shot blasting to form a friction surface, it is necessary to process many steel products to be joined which are available in a wide variety, thus resulting in an increase in labor and time and raising the processing cost, which poses a problem. Further, even if a method including shot blasting is used to make different the hardness and the roughness of a friction surface to raise friction coefficients of the friction surface, there still remains a wide variance in the thus obtained friction coefficients. Consequently, there is not a substantial improvement in the upper limit of the friction coefficients available in design as expected, thus, resulting in a smaller cost-effectiveness, which poses another problem.
An object of the present invention is to provide a friction-joining steel plate and a friction-joining structure, which are applicable to a wide variety of friction joining portions at a low cost and also able to raise frictional resistance reliably, thereby realizing a rational design.